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Modification of Gold Nanoparticle Composite Nanostructures Using Thermosensitive Core-Shell Particles as a Template Daisuke Suzuki and Haruma Kawaguchi* Faculty of Science & Technology, Keio University, Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Received February 18, 2005. In Final Form: June 10, 2005 We report the formation of novel thermosensitive hybrid core-shell particles via in situ synthesis of gold nanoparticles using thermosensitive core-shell particles as a template. The template core-shell particles, with cores composed mainly of poly(glycidyl methacrylate) (GMA) and shells composed mainly of poly(N-isopropylacrylamide) (PNIPAM), were synthesized in aqueous medium, and functional groups such as thiol groups were incorporated into each particle. We found that these particles containing thiol groups were effective for the in situ synthesis of gold nanoparticles in long-term storage. The obtained hybrid particles exhibited a reversible color change from red to purple, which originated from the surface plasmon resonance of gold nanoparticles and which was temperature-dependent in the range of 25-40 °C. In addition to their thermosensitive property, the hybrid particles exhibited the unique characteristic of uniform distribution on a solid substrate. The particles obtained by this approach have potential thermosensitive applications such as in sensors and photonic or electronic devices.

Introduction Recently, metal nanoparticles have attracted much attention due to their potential use in photonic, electronic, and magnetic devices.1 Metal nanoparticles have been assembled into one-, two-, and three-dimensional architectures1-5 as well as colloidal aggregates.6-9 The conjugation of gold nanoparticles with functional materials is especially interesting because of their optical properties termed surface plasmon induced by the collective oscillation of electron density.10 Mirkin and co-workers reported that oligonucleotide-attached gold nanoparticles could be used for sensitive colorimetric detection.6 Materials that combine poly(N-isopropylacrylamide) (PNIPAM) and gold nanoparticles have also been reported. PNIPAM is a representative thermosensitive polymer that exhibits a lower critical solution temperature (LCST) in water around 32 °C.11 Tenhu and co-workers reported that PNIPAM produced by controlled radical polymerization was introduced onto gold nanoparticles by a “grafting-to” or a “grafting-from” method.12,13 They revealed that a PNIPAM brush bound to gold nanoparticles showed two* To whom correspondence should be addressed. Address: Graduate School of Science & Technology, Keio University, 3-14-1 Hiyoshi, Yokohama 223-8522, Japan. Phone: +81-45-566-1563. Fax: +81-45-564-5095. E-mail: [email protected]. (1) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem. 2000, 1, 18. (2) Musick, M. D.; Keating, C. D.; Keefe, M. H.; Natan, M. J. Chem. Mater. 1997, 9, 1499. (3) Gittins, D. I.; Bethell, D.; Nichols, R. J.; Schiffrin, D. J. Adv. Mater. 1999, 11, 737. (4) Wang, Z. L. Adv. Mater. 1998, 10, 13. (5) Kiely, C. J.; Fink, J.; Brust, M.; Bethell, D.; Schiffrin, D. J. Nature 1998, 396, 444. (6) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (7) Shenton, W.; Davis, S. A.; Mann, S. Adv. Mater. 1999, 11, 449. (8) Boal, A. K.; Llhan, F.; Derouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746. (9) Maye, M. M.; Chun, S. C.; Han, L.; Rabinovich, D.; Zhong, C.-J. J. Am. Chem. Soc. 2002, 124, 4958. (10) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293. (11) Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81, 6379. (12) Raula, J.; Shan, J.; Nuopponen, M.; Niskanen, A.; Jiang, H.; Kauppinen, E. I.; Tenhu, H. Langmuir 2003, 19, 3499.

phase transitions.14 Another group found that gold nanoparticles coated with thiol-terminated PNIPAM exhibited reversible, temperature-dependent change in optical transmittance.15 Lyon and co-workers reported that PNIPAM gel colloidal crystal co-assembled with gold nanoparticles could be locally interconverted between a glassy and a crystalline structure by using gold nanoparticles as localized heat sources.16-18 Kumacheva and co-workers prepared hybrid microgels with inorganic nanoparticles.19 They obtained, most notably, hybrid microgels with gold nanorods that achieve photothermally modulated volume transitions.20 Other approaches to constructing hybrid materials using metal nanoparticles have been succeeded in the area of core-shell and hollow particles. Akashi and co-workers reported that welldispersed platinum, silver, or Au/Pt bimetallic nanoparticles can be synthesized in situ on polystyrene particles polymerized by dispersion copolymerization using PNIPAM macromonomer.21-23 Caruso and co-workers reported a number of interesting works using a layer-by-layer method for assembling polyelectrolyte sequentially adsorbed particles,24 such as gold nanoparticle-based core-shell and hollow particles.25 They also investigated the optical (13) Shan, J.; Nuopponen, M.; Jiang, H.; Kauppinen, E.; Tenhu, H. Macromolecules 2003, 36, 4526. (14) Shan, J.; Chen, J.; Nuopponen, M.; Tenhu, H. Langmuir 2004, 20, 4671. (15) Zhu, M.-Q.; Wang, L.-Q.; Exarhos, G. J.; Li, A. D. Q. J. Am. Chem. Soc. 2004, 126, 2656. (16) Jones, C. D.; Lyon, L. A. J. Am. Chem. Soc. 2003, 125, 460. (17) Lyon, L. A.; Debord, J. D.; Debord, S. B.; Jones, C. D.; McGrath, J. G.; Serpe, M. J. J. Phys. Chem. B. 2004, 108, 19099. (18) Jones, C. D.; Serpe, M. J.; Schroeder, L.; Lyon, L. A. J. Am. Chem. Soc. 2003, 125, 5292. (19) Zhang, J.; Xu, S.; Kumacheva, E. J. Am. Chem. Soc. 2004, 126, 7908. (20) Gorelikov, I.; Field, L. M.; Kumacheva, E. J. Am. Chem. Soc. 2004, 126, 15938. (21) Chen, C.-W.; Chen, M.-Q.; Serizawa, T.; Akashi, M. Adv. Mater. 1998, 10, 1122. (22) Chen, C.-W.; Serizawa, T.; Akashi, M. Chem. Mater. 1999, 11, 1381. (23) Chen, C.-W.; Serizawa, T.; Akashi, M. Chem. Mater. 2002, 14, 2232. (24) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111.

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Figure 1. Schematic representation of synthesis of thermosensitive hybrid core-shell particles with gold nanoparticles.

properties of ordered structures obtained from particle self-assembly.26 In this study, we developed thermosensitive hybrid core-shell particles via in situ gold nanoparticle formations using thermosensitive particles as a template. Figure 1 shows the schematic route used in this study. To prepare template particles for the in situ synthesis of gold nanoparticles, we chose NIPAM and glycidyl methacrylate (GMA) as monomers. We selected GMA as a monomer to provide a reactive site for the immobilization of functional groups, such as amino or thiol groups, inside the thermosensitive layers. The thiol groups were expected to hold the gold nanoparticles effectively inside the shell. We designed the template structures and compositions so as to yield a variety of hybrid particles whose properties would respond to temperature changes. Here, we report the preparation and characterization of template particles as well as their hybrid particles based on gold nanoparticles, especially those that reversibly change color according to temperature. Experimental Section Materials. NIPAM was kindly given by Kojin Co. and recrystallized from hexane:toluene (1:1 on a volume basis). GMA was purified by distillation under a reduced pressure to remove inhibitors. N,N′-Methylenebisacrylamide (MBAAm) was used without further purification. Azobisamidinopropane dihydrochloride (V-50) was used without further purification. 2-Aminoethanethiol (2-AET, Tokyo Kasei Kogyo Co., Ltd.) and (()dithiothreitol (DTT) were used without further purification. Chloroauric acid (HAuCl4) and sodium borohydrate (NaBH4) were used as received. The water used in all experiments was from a Milli-Q reagent water system (Millipore). All reagents were purchased from Wako Pure Chemical Industries, Ltd., unless otherwise noted. Preparation of the NIPAM-co-GMA (NG) Particles. To obtain the template particles, NIPAM, GMA, and MBAAm were copolymerized with different feed ratios in a soap-free aqueous medium using V-50 as an initiator. A mixture of 0.2 g of NIPAM, 2.2 g of GMA, 0.04 g of MBAAm, and 90 g of water was put into a 200-mL three-neck, round-bottom flask equipped with a stirrer, a nitrogen gas inlet, and a condenser. Nitrogen gas was bubbled into the mixture to purge oxygen. The system was kept at 70 °C in a water bath. A total of 10 g of water containing 0.06 g of V-50 was added to the flask to initiate polymerization. To cover the surface of the particles with PNIPAM, a total of 10 g of water containing 0.6 or 1.8 g of NIPAM was added to the flask at 1 h after the initiation of polymerization, which was continued for a further 3 h. The obtained particles (NG particles) were purified by centrifugation and then washed four times with water. The epoxy groups of the NG particles were allowed to react with 2-AET and DTT to introduce the amino and thiol groups, respectively, to the particles (NG-NH2 or NG-SH particles, respectively). A mixture of 0.4 g of NG particles, 0.78 g of 2-AET or 0.63 g of DTT, and 70 g of water was put into a 100-mL glass vial with stirring at room temperature, and the pH was adjusted to 11.0. The reaction was continued for 24 h. The obtained particles were purified by centrifugation using the method described above and were subjected to dialysis for a week. (25) Liang, Z.; Susha, A.; Caruso, F. Chem. Mater. 2003, 15, 3176. (26) Salgueirin˜o-Maceria, V.; Caruso, F.; Liz-Marza´n, L. M. J. Phys. Chem. B 2003, 107, 10990.

Synthesis of Gold Nanoparticles inside Thermosensitive Core-Shell Particles. A mixture of 10 mg of template particles, 5 mg of HAuCl4, and a total of 10 mL of water was poured into a 50 mL glass vial at room temperature and agitated for 30 min. A total of 1 mL of aqueous solution of 0.25 mg NaBH4 was added to the vial dropwise. After the addition of NaBH4, the mixture was allowed to stand for 30 min, after which the particles were purified by dialysis for a week. Characterization. Approximately 2 µL of the diluted particle suspension was dried on a carbon-coated copper grid (Okenshoji Co., Ltd.) and observed by field emission transmission electron microscopy (TEM, TECNAI F20, Philips Electron Optics Co.) operated at 200 kV. The hydrodynamic diameters of particles were determined by dynamic light scattering (DLS) using a laser particle analyzer system (PAR-3, Otsuka Electronic Co.). The incident wavelength of the He-Ne laser was 632 nm, and the measurement angle was 90°. UV-visible absorption data were recorded on a Hitachi U-2001 spectrophotometer. Before measurement, the samples for the DLS and UV-visible absorption experiments were allowed to equilibrate at each temperature for 10 min. Elemental analysis of the dried particles for carbon, hydrogen, nitrogen, oxygen, and sulfur was conducted by Vario EL (Elementar Analyzensysteme GmbH).

Results and Discussion Preparation and Characterization of NG Particles. Monodisperse NG particles were prepared in soapfree aqueous medium. Given the different reactivity ratios of NIPAM and GMA (0.39 and 2.69, respectively),27 GMA tends to be consumed faster than NIPAM. Therefore, before the NIPAM solution was added, the interior of the core particle was supposed to be rich in PGMA, whereas the exterior was supposed to be rich in PNIPAM chains. In the synthesis of the shell layer, the collapsed core particles serve as nuclei for further polymerization. This leads to the preferential growth of existing particles over the nucleation of new ones. NIPAM solutions were poured into the flask at 1 h after the initiation. At this stage, almost all of the GMA () 90%) was polymerized. As a result, the shell was composed almost entirely of NIPAM and a small amount of GMA. GMA allows the structure to be modified so as to introduce amines, carboxylic acids, and thiol compounds. We introduced amino or thiol functionalities using 2-AET or DTT, respectively, to the NG particles by the reaction between thiol groups and epoxy groups. In each case, the amounts of introduced compounds were calculated from the sulfur content measured by elemental analysis. The obtained sulfur contents for the modified particles were about 2.6 wt % for 2-AET and 2.7 wt % for DTT. Almost the same values were obtained regardless of the shell thickness. The use of sulfur content and the amount of GMA in the shell allowed us to estimate the amounts introduced in the shell. The amounts of introduced compounds were about 10 mol % against the amount of GMA in the shell, although these estimates contain uncertainties. Figure 2 shows the temperature dependence of the hydrodynamic diameters of the NG particles (NG1, NG2) and of the NG particles containing thiol or amino groups (27) Virtanen, J.; Tenhu, H. J. Polym. Sci. Part A: Polym. Chem. 2001, 39, 3716.

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(NG1-SH, NG2-SH, NG1-NH2, and NG2-NH2) in aqueous dispersions. It should be mentioned that the DLS measurement was performed only at 90°; consequently, the obtained hydrodynamic diameters are not very accurate and are used only as rough estimates in this study. However, the comparison of DLS data is still believed to be meaningful for the analysis of temperature-dependent colloidal properties. In all experiments using DLS measurements, no salt was added and the pH value was not adjusted. In the present study, the pH value was around 6. The dispersity, the ratio of the weight-average diameter to the number-average diameter, was less than 1.03 in all cases. The NG2 particle diameters in the swollen state are much larger than those of the NG1 particles due to the additional feed of NIPAM for the former. The NG particles showed a volume phase transition of PNIPAM at around 32 °C, but the response was dull, especially in the NG1 particles. This is because the shell’s network structure restricted the chain motion and retarded the response.28,29 The introduction of thiol groups to each NG particle was successful, and their diameters in the swollen states were less than those before incorporation of the thiol groups. This fact suggests that thiol groups incorporated in the shell layer might be reduced to disulfides during the reaction or dialysis, and consequently the shell volume is decreased, since disulfides behave as crosslinkers. In general, particles whose surfaces are covered with thiol groups are unstable due to their reactivities among them. In this case, however, we successfully obtained highly dispersed particles, because NG particles in the swollen states showed high steric hindrance from well-solvated PNIPAM chains at the particles’ outermost layer. Although the thiol groups were allowed to react with each other inside the shell, they could not react with the thiol groups of neighbor particles. The particles were also stable in the shrunken states, which indicated that the thiol groups were fully covered with dehydrated PNIPAM chains. On the other hand, in the case of amino groups introduced into NG particles, the NG1-NH2 and NG2-NH2 did not shrink at around 32 °C, and their diameters did not plateau in the shrunken state between 20 and 40 °C. This is because protonation of amino groups under pH 6 increases the hydrophilicity of polymer chains

as well as their repulsiveness, thus preventing their collapse up to higher temperatures. This is a common phenomenon observed with ionic-group-carrying PNIPAM.30 Further evidence of the incorporation of amino groups was the increment of diameter as temperature increased from 20 and 40 °C. The effect of ionic monomer incorporation into PNIPAM on the swelling behavior was studied by some researchers and attributed mainly to osmotic pressure or the Donnan potential due to counterions;31 alternatively, it has also been attributed to repulsion between amino groups.32 The control experiments, using the core-shell particles with a PGMA core and a homo-PNIPAM shell, showed no shift in the transition temperature. This indicated that the shells of the NG particles shown here were composed of both PNIPAM and PGMA. In Situ Formation of Gold Nanoparticles. We synthesized gold nanoparticles using a variety of NG particles as templates. NG particles have large volumes in their shells, which were composed of polymer chain networks and large amounts of water. Therefore, the nucleation and growth of gold nanoparticles proceeded successfully in the shells’ empty spaces. First, the dispersion of NG particles and precursor anions of AuCl4- were mixed at several weight ratios. At this stage, the dispersions were yellowish because they originated from the precursor ions. After the NaBH4 was added, the dispersions became dark red immediately. In the control experiments without template particles, gold nanoparticles were not synthesized, and only large black aggregates were sedimented. These experiments revealed that NG particles behaved as stabilizers for gold nanoparticles. However, the particles prepared from NG1 and NG2 particles were not stable during long-term storage. They seemed to be stable during and after the synthesis, but flocculated during dialysis for purification. We therefore conducted further investigations using different types of NG particles, such as NG particles with thiol or amino groups, as templates. As a result, we obtained welldispersed hybrid particles using thiol groups incorporated into particles as templates (NG1-SH-Au and NG2-SHAu particles). These results indicate that the gold nanoparticles in the latter case were fixed much more strongly than those in the former case due to the affinity of the thiol groups to gold nanoparticles. When amino-functionalized particles (NG1-NH2 and NG2-NH2) were used as templates, the obtained particles (NG1-NH2-Au and NG2NH2-Au particles) were as stable as thiol-functionalized ones, but they flocculated while we accessed the thermosensitivity of dispersion. As a result, we concluded that the thiol groups were useful for holding the gold nanoparticles inside the shell and to yield hybrid particles for long-term storage and applications. Figure 3 shows TEM images of hybrid NG1-SH-Au and NG2-SH-Au particles. Parts A and B in the figure show TEM views of NG1-SH-Au particles dried at room temperature on a carbon-coated copper grid. Whereas they did not show a uniform distribution, the NG1-SH particles without gold nanoparticles did, and the gold nanoparticles looked as if they were attached to the core particles. This is attributable to the physical cross-linking of polymer networks with the gold nanoparticles. On the other hand, parts C-F in the figure are TEM views of NG2-SH-Au particles dried at room temperature (parts C and D) and at 50 °C (parts E and F). Unlike the NG1-SH-Au particles,

(28) Ohshima, H.; Makino, K.; Kato, T.; Fujimoto, K.; Kondo, T.; Kawaguchi, H. J. Colloid Interface Sci. 1993, 159, 512. (29) Kato, T.; Fujimoto, K.; Kawaguchi, H. Polym. Gels Networks 1994, 4, 307.

(30) Maeda, Y.; Yamamoto, H.; Ikeda, I. Langmuir 2001, 17, 6855. (31) Ferna´ndez-Nieves, A.; Ferna´ndez-Barbero, A.; Vincent, B.; de las Nieves, F. J. Macromolecules 2000, 33, 2114. (32) Jones, C. D.; Lyon, L. A. Macromolecules 2003, 36, 1988.

Figure 2. Temperature dependence of the hydrodynamic weight-averaged diameter of NG particles and their modified particles in aqueous dispersions measured by dynamic light scattering (temperature-raising process).

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Figure 3. TEM views of thermosensitive hybrid particles with gold nanoparticles deposited on carbon-coated copper grid. NG1SH-Au particles dried at room temperature (A, B). NG2-SH-Au particles dried at room temperature (C, D), and at 50 °C (E, F). Each pair of images was taken at a different magnification.

Figure 4. Temperature dependence of the hydrodynamic weight-averaged diameter of hybrid particles in aqueous dispersions measured by dynamic light scattering (temperatureraising process).

the NG2-SH-Au particles were uniformly distributed on the substrate (part C). The close-up (part D) shows that gold nanoparticles were located around the core particles and were well isolated from each other. The area containing the gold nanoparticles coincided with the area covered by the hydrodynamic diameter of the NG2-SH-Au particles at 20 °C (see Figure 4). In contrast, the particles dried in the shrunken states were distributed randomly on the substrate, and the gold nanoparticles were localized on the core particles, as were the particles shown in part B (see parts E and F). This phenomenon is related to the excluded volumes of the particles. The particles shown in parts C and D of Figure 3 were settled, retaining their own excluded volume, whereas those in the other samples were not. Our group previously reported similar particle arrays using hairy particles.33,34 We did not observe large (33) Tsuji, S.; Kawaguchi, H. Langmuir 2004, 20, 2449. (34) Tsuji, S.; Kawaguchi, H. Langmuir 2005, 21, 2434.

aggregates of gold nanoparticles or independent gold nanoparticles from the TEM views in the present study. The average size of the gold nanoparticles prepared under this preparation condition was about 10 nm. The size and shape of a gold nanoparticle depended on the preparation conditions, such as the concentrations of HAuCl4 and NaBH4, and the times of the treatment cycles. We concluded that the in situ synthesis of gold nanoparticles was successful, and that it is an easy method to create a nano-architecture composed of organic/inorganic composite materials. Optical Properties of Hybrid Particles. From the results of Figure 3 (compare part C with part E), we found that the environment around the gold nanoparticles when the shell is swollen is quite different from that when the shell is in a shrunken state. We therefore conducted further investigations into the hybrid particles’ temperature-dependent optical properties. Figure 4 shows the temperature response of the hydrodynamic diameters of the hybrid particles. We confirmed that this response did not change after the hybridization. In the case of the NG1-SH-Au particle, the diameter in the swollen state was smaller than that of the nonhybrid particle. This indicated the existence of physical cross-linking between the polymers and the gold nanoparticles. In contrast, the diameter of the NG2-SHAu particle in the swollen state was larger than that of the nonhybrid particle. The difference might be attributable to the difference in polymer structures or to the amount of attached gold nanoparticles in each shell. However, we cannot draw a conclusion. Figure 5 shows the absorption spectra of the hybrid particles depending on temperature. For NG1-SH-Au particles (part A), an absorption peak originated from surface plasmon resonance was measured at 528 nm at 20 °C. The intensity and position of the absorption peak measured at 40 °C, at which the shell was shrunken, were almost the same as those of the swollen particle. In the case of NG2-SH-Au particles (part B), an absorption peak was measured at 522 nm at 20 °C. The position of the

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Figure 6. The dispersion of NG2-SH-Au particles at room temperature (A) and at 40 °C (B).

A is an opaque red, whereas part B is an opaque purple. The difference results from the interparticle close-contact coupling of surface plasmon resonance among gold nanoparticles.1 The distances between the gold nanoparticles differ quite a lot between the swollen and shrunken states of the hybrid particles (see parts D and F of Figure 3). Although the gold nanoparticles did not attach to each other when the hybrid particles were in the shrunken state, the gold nanoparticles were localized at the core particle surfaces and the interparticle distances were extremely short. As a result, we obtained temperaturesensitive hybrid core-shell particles with gold nanoparticles. These temperature-dependent color changes are similar to those using gold nanoparticles with oligonucleotides or thermosensitive polymers.6,15 The particles shown in this article have the advantages of easy handling for constructing two- or three-dimensional structures. Further investigations are under way, focusing on controlling the template structures and compositions and on applying other inorganic materials. Conclusions

Figure 5. UV-visible extinction spectra of NG1-SH-Au particles (A) and NG2-SH-Au particles (B, C) measured at both 20 and 40 °C. Part C is a detailed examination at different temperatures.

peak measured at 40 °C was red-shifted and observed at 533 nm. The intensity increased in the visible region due to the Rayleigh scattering of the shrunken particles. Part C of Figure 5 shows in detail the temperature dependence of the NG2-SH-Au particle spectra. Interestingly, these transitions in the NG2-SH-Au particles were completely reversible (10 cycles), indicating that the gold nanoparticles did not attach to each other in the shrunken state of the hybrid particle. Figure 6 shows the colors of the dispersion containing NG2-SH-Au particles at 25 (part A) and 40 °C (part B). We can easily distinguish the colors between them: part

We have synthesized thermosensitive hybrid core-shell particles with gold nanoparticles generated in situ. The hybrid particles exhibited a reversible, temperaturedependent color change. We used several thermosensitive core-shell particles as templates. The template particles with thiol groups were effective for forming gold nanoparticles in situ for long-term storage. Besides their thermosensitive property, the hybrid particles exhibited the unique characteristic of uniform distribution on a solid substrate. The particles obtained by this approach have potential thermosensitive applications such as in sensors as well as in photonic or electronic devices. Acknowledgment. This work was supported by a Grant-in-Aid for the 21st Century COE program “KEIO Life-Conjugated Chemistry” from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. LA0504356